PhD Projects and Thesis

PhD projects are offered and supported by any one of the institutes in the Research School. Supervising scientists not affiliated with one of the partner universities share the supervision with a faculty member. The thesis will be written in English; students are expected to publish at least two papers in refereed journals during their PhD period.

A Thesis Advisory Committee (TAC), to be named at the beginning of the thesis work, guides the student through the preparation period and monitors her/his work. The TAC consists of the thesis supervisor and one or two other senior staff members of the institutes involved in the research school. At least one of the members must be a regular professor at the university at which the student is inscribed.

The following list gives an overview of the variety of research topics, but it is not complete. Some projects may already be taken, and others not mentioned will be available.

Max-Planck-Institut für Sonnensystemforschung

Solar Interior

Solar Atmosphere

Heliosphere and Solar-Terrestrial Relations

Planetary Interiors and Surfaces

Planetary Atmospheres and Magnetospheres

Small Bodies

Instrumentation

Technische Universität Braunschweig

Universität Göttingen

Max-Planck-Institut für Sonnensystemforschung

Solar and stellar oscillations

Contact: Laurent Gizon

Solar-like oscillations are caused by sound waves, excited by turbulent convection, that are trapped within a star. The purpose of helioseismology is to observe and interpret solar oscillations to learn about the internal structure and dynamics of the Sun. The data from the MDI telescope aboard the ESA/NASA SOHO satellite have yielded numerous exciting results. In particular, the validity of the basic standard model of stellar structure has been confirmed, and rotation has been mapped as a function of depth and latitude in the convection zone. Current studies focus on new techniques of local helioseismology to produce three-dimensional maps of the subphotospheric flows and temperature inhomogeneities. Possible thesis topics include the development and implementation of diagnostics tools of local helioseismology to probe the internal structure of sunspots and the dynamics of convective flows. Below is a map of horizontal flows around a sunspot, 1 Mm beneath the solar surface.


Helioseismology is a growing field of research, with several missions planned for the future (HMI, Solar Orbiter). In addition, solar-like pulsations have recently been detected on other stars than the Sun from ground-based observatories. It is hoped that high-precision stellar seismology will become a reality with the ESA space mission COROT.

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Near surface convection

Contact: Laurent Gizon, Aaron Birch

Inverse methods allow us to use observed wave travel-time shifts to infer subsurface physical conditions. One particular target for inversions is near-surface supergranulation-scale convection. For example we can detect the subsurface extension of the moat flow around sunspots as well as the subsurface supergranulation flow pattern. Flow maps of this type will help us study the connections between subsurface flows and magnetic field evolution in the photosphere.

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Deep convection

Contact: Laurent Gizon, Aaron Birch, Robert Cameron

Convection is the mechanism by which energy is transported through the outermost 30% of the Sun and plays an important role in models of stellar structure and evolution. In the past, our understanding of deep solar convection has been based on theoretical models and numerical simulations. Recently, however, a new method for using helioseismology to study the statistical properties of convection in the deep solar convection zone has been developed. We have found that current numerical simulations of large-scale convection carried out with the commonly employed Anelastic Spherical Harmonic (ASH) code substantially overestimate horizontal convective velocities. In addition, the upper limit provided by helioseismology is below the theoretical lower limit on convective velocities that follow from demanding that the convective Reynold's stresses be strong enough to maintain the observed solar differential rotation and meridional flow. Thus helioseismology observations appear to be a substantial challenge for models of subsurface convection.

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Sunspot seismology

Contact: Laurent Gizon, Hannah Schunker, Robert Cameron

Sunspots are an important observational element of the solar magnetic cycle: they are the most notable product of the emergence of flux from beneath the solar surface. They are well observed at the surface, but where they come from and how they are attached to, or disconnected from deeper structures remains a matter of speculation. Helioseismology is the only tool with the capacity to move from speculation to observational knowledge.

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What determines the strengths of solar cycles?

Contact: Laurent Gizon, Robert Cameron, Manfred Schüssler

The solar dynamo produces magnetic activity that rises and falls in an approximately 11 year cycle. Some of the cycles are very active; some are weak. What causes this variation in the strength of cycles?

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Asteroseismolgy, rotation and exoplanets

Contact: Laurent Gizon, Hannah Schunker

An important goal of asteroseismology is to accurately determine the properties of individual stars. Asteroseismology constrains stellar models and informs our understanding of how stars and their planetary systems form and evolve.

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Stellar modelling

Contact: Laurent Gizon, Aaron Birch

The transport of angular momentum in stellar radiative zones is not well understood. Angular momentum is transported in stellar radiative zones by mechanisms such as meridional circulation, turbulent viscosity, magnetic fields and internal gravity waves. Such mechanisms have been modeled with several prescriptions and applied to stellar evolution codes. Transport by meridional circulation and turbulent viscosity, in particular, has been used to study the rotation profiles of main-sequence and red-giant stars. Until recently, however, direct information on the core rotation was not available. Thus, direct comparisons with observation were not possible. This has recenttly changed opening up a number of interesting possibilities.

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Solar and stellar magnetohydrodynamics

Contact: Robert Cameron (cameron AT mps.mpg.de)

Magnetohydrodynamics (MHD) describes the dynamical interaction between flows and magnetic fields in a collisionally dominated plasma. The work of the MHD group at MPS is mainly concerned with MHD processes in the interior and atmosphere of the Sun and other stars. Using large-scale numerical simulations and analytical tools, this work aims at understanding the origin of solar and stellar magnetism as well as its rich variety of manifestations in the form of magnetic activity.

The research areas of the MHD group at MPS include

Our work is closely connected to solar and stellar observations.

Group home page: http://www.mps.mpg.de/projects/solar-mhd/

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Solar magnetic field measurements

Contact: Andreas Lagg, Sami K. Solanki, Group: Ground Based Solar Observations (GBSO)

The magnetic field is the basic quantity that causes all the various active phenomena on the Sun (flares, mass ejections, sunspots, the hot corona, etc.) The Group Ground based solar observations (GBSO) specializes in state of the art measurements of the Sun's magnetic fields and is thus at the heart of solar physics. Techniques including the Zeeman effect in spectral lines are used to determine the 3-D structure of the magnetic field vector, the velocity field, temperature, etc. An advanced inversion code is used to derive these images and extract the huge amount of information on the structure of the solar atmosphere and its magnetic field from the observed high resolution spectra. The results are ideally suited to verify and improve theoretic models of all kinds of solar phenomena. The GBSO group offers manifold possibilities to work between theory and data analysis. Your work as a PhD student will benefit from

- state of the art instrumentation for obtaining data (eg. the MPS/IAC Tenerife Infrared Polarimeter II),

- the possibility to carry out observations with one of the worlds leading solar telescopes in exciting locations (eg. the Vacuum Tower Telescope on Tenerife or the Swedisch Solar Telescope on La Palma),

- the access to data from space-born telescopes like Solar-B or SOHO,

- the presence of high quality analysis software for spectropolarimetric data and

- the close connection with theoretic modeling of solar observables.

The Vacuum Tower Telescope on Tenerife, harboring our Infrared Polarimeter


Polarimetric signal of a magnetic sensitive spectral line: The Zeeman effect increases the splitting of the sublevels of an atomic transition. The magnetic field direction can be inferred by analyzing the linear and circular polarization state of the spectral line (Stokes parameters: I, Q, U and V).


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Non-MHD effects in the solar atmosphere

Contact: Manfred Schüssler, Eckart Marsch

Particle density, collision rate and ionization degree decrease strongly with height in the lower solar atmosphere, so that the application of simple one-fluid magnetohydrodynamics becomes questionable in regions around the temperature minimum and above. An exploratory study is intended to investigate under which conditions effects like ambipolar diffusion and the Hall effect become relevant and when multi-fluid approaches should be used.

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Modeling the structure and dynamics of the solar corona

Contact: Hardi Peter ( peter AT mps.mpg.de )

The cool Sun is surrounded by a hot outer atmosphere. This million K hot corona is heated through conversion of magnetic energy, even though the details of this process are not well understood. In the "coronal dynamics" group we investigate the dynamics, structure and heating of the corona with the help of numerical model, e.g. 3D magnetohydrodynamics (MHD) models that account for the interaction of the magnetic field and the plasma. Using high-performance computing we simulate the processes in the corona and synthesize the coronal emission in the extreme UV and X-rays. This allows us to directly compare out models with real observation of the Sun. In this framework several topics are available for a PhD thesis, including the implementation of new physical processes into the numerical models, parametric studies of the response of the corona to the driving from the solar surface, or the synthesis of emission lines for comparison with actual space observations, e.g., from Hinode, SDO or the upcoming IRIS mission.

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Magnetohydrodynamic plasma simulation of structure and dynamics of the Solar atmosphere

Contact: J. Büchner

The Solar magnetic field is a major factor organizing the release of plasma and wave energy from the Sun into the heliosphere. Using modern magnetographic techniques, in part under development at the MPAe, the full Solar vector magnetic field information is becoming available at higher resolution than ever. Since direct measurements of the solar magnetic fields are restricted to a thin layer close to the Solar surface the fields have to be extrapolated to the Solar atmosphere (Figure). To understand the consequences of these fields for the heliosphere, however, one has to consider also the plasma of the Solar atmosphere in its interaction with the magnetic fields. For this purpose the PHD project aims at the development of computer models, starting with the search for appropriate equilibria. The combination of equilibrium configurations with the measured magnetic fields should provide realistic plasma models which then will be used to simulate, this way trying predict the evolution of Solar plasmas and fields. It is planned to compare the results of this modelling effort with existing observations of the SOHO spacecraft where the MPAe is heavily involved, in the future with the STEREO and other Solar missions.

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Kinetic simulation of magnetic reconnection and helicity evolution

Contact: J. Büchner

Reconnection is the main physical process of plasma heating and particle acceleration by utilizung the energy stored in magnetic fields In the "Theory and simulation of Solar system plasmas" (TSSSP) group appropriate methods are being developed to simulate the dynamics including plasmakinetic effects. The latter have to be taken into account since in the collisionless and complex space plasmas direct interaction between particles are rare and collective effects therefore prevail. The resulting physical effects are highly nonlinear. They can be best described by numerical simulation techniques. In the TSSSP group at MPAe different types of codes have been developed which can be applied using powerfull parallel computing facilities. One of the PHD projects in this direction is to investigate the consequences of three-dimensional reconnection (Figure) for the dynamics of the Solar atmosphere, particle acceleration into the Solar wind and radiation. A special role which has to be investigated plays the magnetic helicity, a remotely measurable quantity, its creation, evolution, transport since it might enable us to predict Solar eruptions and their consequences for the space wheather at Earth.

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Diagnostics of the solar corona via vacuum ultraviolet spectroscopy and imaging

Contact: Werner Curdt, Luca Teriaca


The solar VUV spectrum from 50 nm to 160 nm recorded by SUMER

It was the combined development of spectroscopy and atomic physics that led, at the end of the thirties of last century, to the discovery of a hot (over one million K) corona above the cool (5780 K) solar photosphere. Since then, the development of the available instrumentation and atomic physics calculations have provided an increasing sophisticated and powerful tool for the study of the solar and stellar coronae.

The low density (about 109 electrons cm-3), optically thin coronal plasmas emit a spectrum dominated by emission lines (generally due to exitation by electron collisions and subsequent spontaneous decay) and continuum. This radiation is function of the thermodynamic state of the emitting source (i.e., density and kinethic temperature of electrons and element abundances). Thus, the knowledge of the atomic processes leading to the formation of lines and continuum coupled to the measurement of the emitted radiation allows the characterization of the emitting source. Spectroscopy, in particular, besides isolating specific emission lines, allows the measurements of plasma motions on scales larger (bulk motions leading to Doppler shift of the entire line) and smaller (modification of the line profile) than the spatial resolution, providing information on the dynamics and mass transport in the atmosphere. Moreover, being the coronal plasma forced to move along the magnetic field lines, the radiance and velocity patterns trace the magnetic field well above the photosphere, in regions where cannot be measured directly. The Figure on the side shows an example of velocity maps obtained by SUMER. In this case the target was a large sunspot, but such studies can be performed on any other region of the Sun.


SUMER line of sight velocity maps above a large sunspot. Top: Si IV (80000 K). Bottom Ne VIII (630000 K).

The coronal emission falls almost entirely in the vacuum ultraviolet (VUV: 10 to 200 nm). Thus, the study of solar and stellar coronae requires space instrumentation. The VUV group at MPS is very active on this research topic. It has built and operates the SUMER spectrograph aboard the SOHO spacecraft and is involved in several other important present (Stereo) and future (Solar Orbiter) solar space missions.

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Chromospheric and coronal magnetic fields

Contact: Thomas Wiegelmann, B. Inhester

The solar magnetic field couples the solar interior with the photosphere and corona where it drives heating processes and eruptive phenomena. In the corona the magnetic energy is quasi-statically built up until it exceeds by far the energy stored in a potential magnetic configuration. This excess energy is intermittently released in parts by large eruptive phenomena, e.g. coronal mass ejections, flares and eruptive prominences, but also through small ones, such as explosive events and nano- and micro flares which are probably central for heating the solar corona. Knowledge regarding the coronal magnetic field therefore plays a key role for obtaining a better understanding of these phenomena. Most measurements of the magnetic field vector are restricted to the photosphere, so that the magnetic field needs to be extrapolated from there into the corona. High resolution vector magnetograms are available from ground based (SFT, SOLIS) and space born instruments (Hinode, from 2009 on SDO). Codes for modelling the chromospheric and coronal magnetic field have been developed at the MPS.

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Coronal holes

Contact: Thomas Wiegelmann, Bernd Inhester

  

Coronal holes are regions with a strongly reduced emissivity in coronal lines, while the emissivity in cooler transition region radiation is only slightly reduced compared with the quiet Sun. Coronal holes are characterized by the dominance of one magnetic field polarity on an extended surface region which feeds open magnetic flux into the heliosphere on large scales. On small scales, however, also closed magnetic loops may exist of in coronal hole regions. The average length and height of these loops is much smaller than in the normal quiet Sun regions. The plasma stored on these low level loops is responsible for the cool transition region radiation out of coronal holes while the the absence of long coronal loops explains the absence of EUV ad X-ray emissions. An open question is why and how do coronal holes occur and vanish? Current theories suggests that coronal holes are generated prefrentially close to the boundary of decaying active regions. Magnetic field line reconnection is assumed to play an important role within this process. It is also unclear, how a coronal hole finally vanishes and is converted into a normal quiet Sun region. The aim of this project is to study the temporal evolution of coronal holes and the nearby regions. Photospheric magnetic field data are currently available from ground based observations and space missions (SOHO, Hinode and from 2009 with SDO). With these means, the magnetic field topology should be analyzed be in closed connection with the plasma radiation observed with the two STEREO spacecraft and the coronal plasma flow via SOHO/SUMER Doppler maps.

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How to combine coronal stereoscopy, tomography and magnetic field reconstruction?

Contact: Thomas Wiegelmann, Bernd Inhester

The two STEREO-spacecraft observe the Sun from two vantage points in space. Coronal EUV-images from these two viewpoints allow to derive the 3D-structure of coronal plasma loops and polar plumes. Until now the 3D reconstruction has been done mainly by classic and magnetic stereoscopy. A novel approach is to apply tomographic techniques for the 3D-reconstruction of coronal structures, which is designed in particular for the optical thin coronal plasma. Due to the high conductivity of the coronal plasma, the EUV-loops also outline the coronal magnetic field and allow to test coronal magnetic field models. The coronal modelling approaches, 3D-stereoscopy and tomography all contain measurement errors and reconstruction bias. Within this work it is planned to combine stereoscopy and magnetic modelling in order to derive the solar coronal magnetic field and plasma self-consistently from multi spacecraft observations.

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Magnetic modelling of the interface region between solar photosphere, chromosphere, transition region and corona

Contact: Thomas Wiegelmann, Bernd Inhester

The solar magnetic field is key to understanding the physical processes in the solar atmosphere. Unfortunately, we measure the magnetic field vector routinely with high accuracy only in the photosphere with, e.g., Hinode/SOT. These measurements are extrapolated into the corona under the assumption that the field is force-free. This condition is not fulfilled in the photosphere, but is in the chromosphere and corona. In order to make the observed boundary data consistent with the force-free assumption, we therefore have to apply some transformations before nonlinear force-free extrapolation codes can be legitimately applied. We developed a minimization procedure that uses the measured photospheric field vectors as input to approximate a more chromospheric like field. The procedure includes force-free consistency integrals, spatial smoothing, and an improved match to the field direction as inferred from fibrils as can be observed in, e.g., chromospheric H-Alpha images. While the combination of pre-processing and force-free modelling allows us to reasonably estimate of the chromospheric and coronal magnetic field, we do not fully understand the magnetic field structure between photosphere and chromosphere, where the field is not force-free. A significant part of the energy is stored in this region, however, and we aim for a better understanding of this relatively thin layer. Because this layer contains a finite-beta plasma a force-free magnetic field model is not appropriate here and we need to apply magneto-hydro-static models which include the plasma and magnetic field self-consistently.

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Interstellar dust in the solar system

Contact: Harald Krüger

In-situ dust detectors on board the interplanetary spacecraft Ulysses, Galileo and Cassini have measured interstellar dust particles in the solar system. The grains originate from the Local Interstellar Cloud (LIC) outside our solar system and they act as tracers of the physical conditions in the LIC. In the inner solar system the interstellar dust stream is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the interplanetary magnetic field. Cosmic dust is an important player in many astrophysical processes like, for example, proto-planetary accretion discs and the formation of planetesimals. The physical processes involved in cosmic dust are of far-reaching significance. The PhD thesis deals with the interpretation of the Ulysses interstellar dust measurements and modelling the grain interaction with the heliosphere, in particular with the time-varying interplanetary magnetic field. The implications for the physical conditions of the LIC shall be assessed.

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Internal structure of Mercury from tides observed by laser altimetry

Contact: Ulrich Christensen

Solar body tides on Mercury cause elevation changes of up to 1.5 m during one solar day. The knowledge of the planet's response to tidal forcing, expressed by the Love number h, helps to constrain the interior structure, such as the size of the core and possibly that of a solid inner core. A laser altimeter on board the Mercury Planetary Orbiter of the space mission Bepi Colombo will determine the surface elevation within a meter or better. A strategy for analyzing the altimeter data to get the best possible estimate of h is to be developed and must be tested by modeling with synthetic data, by evaluation of calibration data of the laser altimeter, and by studying the position uncertainties of the Mercury Planetary Orbiter as measured by the MORE radio science experiment. The implications of statistical and systematic uncertainties in h for structural models of Mercury will be assessed.

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Modeling of Mercury's dynamo

Contact: Ulrich Christensen

Mercury has an internal magnetic field that could be generated by a convection driven MHD-dynamo in the liquid part of its core. Possibly only a thin outer shell of the core is still liquid, which might explain the relative weakness of Mercury's field compared to other dynamo-generated planetary fields. As an alternative, the dynamo may not be self-sustained, but is assisted by currents arising from thermoelectric effects at the core-mantle boundary. In numerical simulations both self-sustained dynamos with different shell thicknesses and thermoelectrical dynamos are to be studied in order to develop an understanding for Mercury's peculiar field.

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Modeling mantle convection and the thermal evolution of Mercury

Contact: Ulrich Christensen

Mercury's silicate mantle is only about 600 km thick and overlies a comparatively large core. Mantle convection controls the thermal evolution of the planet and is to be studied in two- and three dimensional Cartesian and spherical models. Such models will make predictions concerning the heat flow at the surface and in the core, thickness of the lithosphere, size of a solid inner core, global shrinking of the planet, topography at the core mantle boundary, and the pattern of convection. These predictions will assist the interpretation of results from the upcoming space missions to Mercury.

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Scaling laws for planetary dynamos

Contact: Ulrich Christensen

The strength and geometry of the various planetary magnetic fields is highly variable and it is not well understood what controls them. Numerical dynamo models can successfully reproduce many observed properties, but the values of some control parameters are far from realistic. Systematic studies covering a decent range of parameters are now possible. They will be employed to derive scaling laws that link properties such as characteristic velocities, heat flow, and magnetic field strength to the control parameters. Such scaling laws, if they can be extrapolated to realistic parameter values, provide a means to explain the differences in planetary magnetic fields.

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Solar signals in the middle atmospheric water vapour and ozone

Contact: Paul Hartogh

Water vapour plays a crucial role in the ozone chemistry of the middle atmosphere of the Earth since it is the source gas for hydroxyl radicals. With increasing altitude in the middle atmosphere the hydroxyl catalytic cycle starts to dominate ozone depletion. The SPARC (stratospheric processes and their role in climate) water vapour assessment of the WMO-World Climate Research Programme reported a 50 to 100 % increase of water vapour in the middle atmosphere since the 1950s which can only partly be explained by anthropogenic influences like the strong increase of methane (which is oxidized to water vapour). We would like to investigate the influence of the solar ultra violet variability on the middle atmospheric water vapour and ozone (see related program: http://www.bu.edu/cawses/germanymay2005.htm). While the total solar irradiance only changes by < 0.1% over a solar cycle, the Lyman-alpha radiation, mainly responsible for water vapour photolysis, typically varies by a factor of 2. Recent theoretical work proposes atmospheric coupling mechanisms that should result in a time delayed effect of solar ultra violet variability on the middle atmospheric water vapour and ozone concentrations. The MPS is running water vapour and ozone microwave spectrometers (see: http://alomar.rocketrange.no/water-vapor.html) covering a period of nearly one solar cycle that provide unique, high quality data sets. The purpose of this PhD project is to analyze these data sets with respect to the context mentioned above and to compare the findings with the model predictions.

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Solar system research with Herschel and SOFIA

Contact: Paul Hartogh and Christopher Jarchow

The Herschel Space Observatory (see: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=16 and http://herschel.jpl.nasa.gov/) will be launched in summer 2007 and will start its observations right away. The MPS as part of the instrument teams plays a leading role in using Herschel for solar system investigations. Specific topics covered by our guaranteed observation times concern the origin of water in the upper atmospheres of the outer planets, the Martian water cycle, the chemistry of the Martian atmosphere, and search for water in distant comets, water excitation and the D/H ratio in comets. Already in 2006 we expect to perform first science flights with SOFIA (see: http://www.sofia.usra.edu/ and http://www.irs.uni-stuttgart.de/aktuelles/). With our hardware contribution to the GREAT-instrument (see: http://www.sofia.usra.edu/Science/instruments/instruments_great.html) we intend to address similar scientific and/or complementary topics by observing solar system objects in the THz range of the electromagnetic spectrum. Depending on the qualification and interests of the applicant(s) the PhD project(s) would either focus on a planetary or cometary topic with some preference for the latter.

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The Martian Atmosphere: Observation and Modeling (MAOAM)

Contact: Paul Hartogh and Alexander S. Medvedev

The atmospheric circulation on Mars is driven by a complex interaction between the radiative, photochemical, and dynamical processes. General circulation models (GCM) of the atmosphere have long been a powerful tool for studying these processes on Earth, and now they are becoming increasingly important for Mars. A new state-of-the-art Martian atmosphere GCM has recently been developed in MPS within the framework of the Mars Atmosphere Observation and Modeling (MAOAM) project. We have PhD opportunities which will focus on improving physical parameterizations in the model, assimilating data from space missions, airborne and ground based measurements, and carrying out numerical simulations to understand the Martian global circulation and its variability.

This figure shows the zonal mean temperature for Ls=90 deg (left). A strong winter polar warming is observed around 60 km. Eliassen-Palm (EP) fluxes (bold red arrow lines) show the propagation trajectories of the wave action, and the thin blue lines present the EP flux divergence (right). The plot shows that the topographically generated waves originate near the surface while the thermal tides are excited in the atmosphere at various heights. In the areas of the strong EP divergence/ convergence, the waves deposit their energy and momentum into the mean circulation. The winter polar warming is strongly affected by the waves.

Equatorial diurnal variation of ozone on Mars during summer solstice calculated with the MAOAM chemistry-transport module.

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Planetary magnetospheres

Contact: Norbert Krupp, Joachim Woch, Elias Roussos

Planetary magnetospheres (the space around a planet where the planetary magnetic field dominates) are large space plasma laboratories where the whole variety of plasma processes can be studied. Especially in-situ measurements of particles, fields and waves aboard spacecraft provide new insights in processes like particle-particle- and wave-particle interactions, particle acceleration or particle precipitation in aurorae. In addition to the interaction between magnetospheric particles and moons, rings and dust the dynamics of these particles, their sources, sinks, and relative abundances could help to significantly enhance the understanding of planetary magnetospheres. The PhD candidates will work on the unique particle data sets of the Galileo and Cassini spacecraft.

Click on image for larger version


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Mapping a comet: Developing photometric models to interpret OSIRIS multi-colour data

Contact: Holger Sierks, Colin Snodgrass

The Rosetta spacecraft will arrive at comet 67P/Churyumov-Gerasimenko in early 2014. The onboard OSIRIS camera system is made up of two cameras with a total of 23 filter combinations, which will be used to take multi-colour images of the comet's nucleus and coma. This PhD project aims to link this colour information to the composition of ices and minerals on the surface, and gasses/ice/dust in the coma, by developing models of what signatures to expect for likely comet components. In addition to differences in composition, the measured brightness of surface components will depend on surface material properties (e.g. grain size) and illumination (local phase angle). The photometric models developed to interpret the OSIRIS data will also take these into account. In the latter part of the project these models will be used to interpret the data from Rosetta, i.e. mapping areas of different composition on the surface. Before arrival at the comet the models will be used to interpret the integrated flux from the comet as observed from ground based telescopes.

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Thermal modeling of cometary nuclei

Contact: Holger Sierks, Jean-Baptiste Vincent

One of the key questions we want to investigate on comets concerns the activity of the nucleus. It has been known for decades that this process is driven by solar illumination, sublimating the ice present at the surface or subsurface of the nucleus. However, it is also known that the activity is usually concentrated in very localized areas of the surface, some of them remaining active even when not illuminated. In support of the Rosetta mission, we are developing a complex model of cometary activity, simulating all physical processes that lead to the creation of "jets" of dust and gas from these active areas. In this context, we want to understand more how the temperature of the surface and subsurface evolves in the active phase of the comet orbit, and we want to develop a complete 3D thermal model of the nucleus, to account precisely for the influence of topography on the diurnal activity.

3D simulations of gas flows around cometary nuclei

Contact: Holger Sierks, Jean-Baptiste Vincent

Another question that needs to be answered is how the local topography affects the activity after the sublimation phase. It is generally assumed that gas will flow around the nucleus and expends into space, and several model have been attempted to describe this, but mainly restrained to ideal cases. We would like to develop here the first full 3D model of gas flows around a cometary nucleus, taking into account the influence of local topography and diurnal variations of temperature associated. This will lead to a better understanding of cometary activity at the surface of the nucleus and the inner coma.

Both students will work in the "Small Bodies working group" at the MPS, where there is already experience and support in observing and modeling physical properties of comets, plus strong implication in several leading instruments of the Rosetta mission. The work is mainly theoretical but students will have access to spacecraft data to test the models.

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Illustration of Rosetta's approach towards comet 67P/Churyumov-Gerasimenko, the lander Philae delivery in Nov 2014 and an example of the envisaged solar wind plasma hybrid simulations


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MIRO observations of Comet 67 P/Churyumov-Gerasimenko

Contact: Paul Hartogh and Christopher Jarchow

The Microwave Instrument for the Rosetta Orbiter (http://www.mps.mpg.de/de/projekte/rosetta/miro/#contribution) is the first radio telescope flown on a "deep space" mission. Since its launch in March 2004, MIRO has already observed a comet and (during the Rosetta Earth fly by in March 2005) provided valuable data about the composition of Earth upper atmosphere. On July 4 of this year (2005) MIRO will observe the increase in water production of Comet 9P/Tempel 1 after the Deep Impact spacecraft has crashed on the comet. MIRO operates in the millimeter and submm domain of the electromagnetic spectrum and observes water and its isotopes, carbon monoxide, ammonia and methanol. The extremely high spectral resolution of 10-7 enables the instrument to exactly determine the spectral line shapes of these molecules and measure their Doppler shifts. Since the distance of the Rosetta spacecraft from the cometary nucleus is so small, MIRO will measure spectral line shapes never seen before and provide completely new clues about processes happening in the inner coma of a comet. MIRO will for instance identify active areas on the cometary nucleus and exactly determine their molecular dependent outgassing rate (via the line strengths) and the velocity distribution (via Doppler-profile/shift). Furthermore MIRO will provide temperature profiles within the inner coma (excitation state of methanol lines) and information about the hydrodynamic of gas jets. The goal of this PhD project is to simulate the molecular spectra we expect to observe with MIRO as a function of the distance from and view on the comet and vice versa to investigate how exactly the input parameters of the simulation can be retrieved from the spectra.

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Solar instrumentation

Contact: Sami K. Solanki, Werner Curdt, Udo Schühle, Eckart Marsch, Rainer Schwenn

The Max-Planck-Institut für Sonnensystemforschung is one of the world's leaders in solar instrumentation. Current projects are contributions to the SECCHI suite of optical instruments to fly on the two STEREO spacecraft (aim: stereoscopy of the Sun), an infrared polarimeter for the solar observatory on Tenerife, a mirror-coronagraph and H-alpha telescope in Argentina, among others.

The Institute is also leading a consortium to build a balloon-borne telescope to obtain the highest resolution images ever taken of the Sun and is playing a leading role in the "Solar Orbiter", the next solar mission of the European Space Agency (aim: explore the Sun's poles and go closer to the Sun than any other mission before).

There are numerous opportunities for PhD theses by experimental physicists interested in developing, designing, building, calibrating, testing and/or employing magnetographs, visible and UV imagers, polarimeters, visible and EUV spectrometers and coronagraphs.

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Wideband-CTS development

Contact: Paul Hartogh and Franz-Josef Ahlers, PTB

MPS has more than 20 years experience in the development of Chirp Transform Spectrometers (CTS). These spectrometers have been used in ground-based, air- and spaceborne millimeter- and submillimetre wave heterodyne experiments. The Chirp Transform (CT) is equivalent to the Fourier Transform. The algorithm describes how to implement a Fourier Transform using linear dispersive pulses and their matched filters. CTSes have several advantages compared to other spectrometer concepts leading to superior measurement accuracy. At the same time a CTS can be designed in a way that mass and power consumption is very small compared to other techniques - an important advantage for space borne applications. The key components in presently used CTSes are the so called surface acoustic wave (SAW) dispersive delay lines (DDLs). The SAW-DDLs are passive signal processing devices. The signal processing capability is achieved by the interaction of surface acoustic waves with sub-´m structures on a piezoelectric substrate. The purpose of this PhD project is to investigate how the maximum CTS- bandwidth of presently 400 MHz can be extended and to find out the physical and technical limits. The project phase I will focus on the design, fabrication and test of new ultra-high time-bandwidth-product DDLs, based on classical materials like LiNbO3, phase II on the investigation of new materials (like for instance TiO2) for this purpose and phase III on testing these new devices in a newly designed CTS. The project will be done in collaboration with PTB ( http://www.ptb.de/index_en.html) and it is assumed that the PhD student will perform his studies to about 50 % of his time there. PTB provides modern high resolution electron beam lithography, metallization and dry etching techniques and is experienced in the production of high frequency SAW, resp. single electron tunneling devices (http://www.ptb.de/en/org/2/25/253/verweis/_sing_electr_transp.htm).

Water vapour spectrum at 22 GHz measured with CTS and retrieved vertical profile of water vapour.

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Technische Universität Braunschweig

Systematic search for regolith structures on planetary bodies

Contact: Jürgen Blum (IGEP, TU Braunschweig), Horst Uwe Keller (MPS)

Modern observations of solar system bodies can reveal information concerning the composition, density and size distribution of the regolith covering planetary, lunar, asteroidal and cometary surfaces. The flyby of ESA's ROSETTA spacecraft by the asteroid Steins in September 2008 shall be used for the determination of mechanical regolith properties, such as the angle of repose, maximum slope angles and the occurrence of avalanches on Steins. The successful participant will work at the MPS within the OSIRIS team; supporting laboratory experiments on regolith behavior under low-gravity conditions shall be carried out in collaboration with the TU Braunschweig team so that the data analysis from the Steins flyby can be based on a strong physical background.

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Mass and energy transport in surface layers of Mars

Contact: Jürgen Blum (IGEP, TU Braunschweig), W. Markiewicz (MPS), Y. Skorov (IGEP)

We focus on the problem whether, where and under which conditions there is water ice on Mars. Research in this area is experiencing a significant rise in the recent years, which is primarily due to the success of recent space missions to Mars. The data obtained by the Gamma-Ray Spectrometer onboard Mars Odyssey were of great importance. This instrument found hydrogen less than one meter beneath the Martian surface. Hydrogen-rich regions are located in areas where ice should be stable. The ice-rich layer beneath the surface was found even at 60 degrees southern latitude. The other important results were delivered by the Phoenix lander. The robotic arm exposed bits of ice by digging a trench into the soil. Crumbs of bright material initially photographed in the trench later vanished, which indicates that they must have been vaporized after being exposed to solar illumination. Finally, laboratory tests aboard the lander have directly identified water vapor in a soil sample delivered by the robotic arm. Thus, there is clear evidence that water ice exists both in the south and north polar regions of Mars. The theoretical prediction that water ice might be conserved very close to the surface was done by Skorov et al. (2001b) for the south polar layered deposits of Mars. Recently the computer model describing the evolution of surface frost and ground ice concentration in a small-scale trench in a north polar region of Mars was developed by Kossacki and Markiewicz (2009). Although encouraging results explaining some details of surface layer evolution were obtained, there are many open questions concerning the transport processes in surface and water cycle on Mars. The main efforts will be focused on investigations of the heat propagation into porous soil and on the long-term stability of ice beneath various non-volatile layers. The important specific feature of the planned research is the close interaction between the laboratory experiments and the extended computer simulations.

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Activity of asteroids and Main Belt comets

Contact: Jürgen Blum (IGEP, TU Braunschweig), H. Boehnhardt (MPS), Y. Skorov (IGEP)

The main goal of this project is investigation of the distant activity of a new class of objects, which have features of both, comets (e.g., gas activity) as well as asteroids (orbital characteristics). These are the so-called main belt comets (Hsieh and Jewitt 2006) or dynamical asteroids with mass loss. Very recently, two independent teams of observers obtained strong evidence that 24 Themis, one of the largest asteroid in the main belt, has a particularly good match for frost of water ice on the surface (Rivkin and Emery 2010). On the basis of observed infrared spectra Campins et al. (2010) draw even stronger conclusions that water ice is distributed across the asteroid surface. It is worth noting that three of the earlier observed main belt comets are members of the Themis dynamical family. Thus, it can be assumed that water ice is a common constituent of these objects and that their observed activities are driven by ice sublimation. As the lifetime of open water ice is rather short at this distance (~3 AU), the steady migration of water through high-porosity media from the asteroidÆs interior to the surface and/or small impacts with asteroid belt boulders puncturing the porous dust layer and expose fresh ice could explain the observations. These recent discoveries will surely trigger a new wave of interest in the modeling of heat and mass transfer in asteroids and comets at large heliocentric distances. The main efforts will be focused on investigations of the long-term stability of ice beneath various non-volatile layers as well as on analysis of collisions of small bodies with porous media. The important specific feature of the planned research is the close interaction between the laboratory experiments, observations and the extended computer simulations.

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Physical behavior of fractal protoplanetary dust

Contact: Jürgen Blum (IGEP, TU Braunschweig)

Fractal dust aggregates play an important role in the formation of planets. When protoplanetary dust particles collide at low velocities, the growing agglomerates are extremely fragile. It is an unsolved problem at which stage of the protoplanetary-dust growth these open-structured agglomerates are being compacted. In this project, we will perform unique laboratory experiments on the collision behavior of fractal dust aggregates. The laboratory at the Institute for Geophysics and extraterrestrial Physics at TU Braunschweig is a world-leading place for such experiments. In close collaboration with modelers from the University of Tübingen, we will investigate under which physical conditions fractal dust aggregates are compactified or fragmented in mutual collisions.

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Collision behavior of micrometer-sized ice particles

Contact: Jürgen Blum (IGEP, TU Braunschweig)

Water ice is a major constituent in the outer Solar System. When the planetary bodies in the outer reaches of the protoplanetary disk formed, collisions among ´m-sized ice particles were frequent. Due to the relatively high surface energy of water ice, it is expected that the sticking efficiency in collisions among water-ice particles is much higher than for the well-investigated silicate particles. With a new and unique experimental setup we intend to study the impact behavior of ´m-sized water-ice particles at cryogenic temperatures to deduce the velocity range in which water-ice particles can grow towards planetesimals or cometesimals. The results will have important impact on the understanding of planet formation in protoplanetary disks.

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Universität Göttingen

Spectral line analysis in sun-like stars

Contact: Ansgar Reiners

Our knowledge about physical properties of stars almost exclusively comes from spectroscopy, and spectroscopic absorption lines carry enormous information about stellar surface structure - e.g. spots, velocities and magnetic fields. Absorption lines are densely packed in cool stars (like the Sun) blurring the information contained in the line shape. Techniques to reconstruct such information are being widely used nowadays but are still in their infancy. One goal is to improve techniques reconstructing stellar absorption profiles, and apply them to high quality astronomical data - more and more of which become available thanks to the growing number of telescopes providing high resolution spectra. On the other hand, individual spectral lines are compared to theoretical models of stellar atmospheres. Improvements of such models can, e.g., include the calculation of magnetic field signatures in rotating stars.

Successful reconstruction of line broadening in a stellar triple system.

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Molecular parameters in very cool stars

Contact: Ansgar Reiners

The most frequent stars in our galaxy are cool stars of spectral type M. Their faintness makes detailed investigation difficult, and surprisingly few is known about their atmospheric structure. At such low temperatures, molecules dominate the visual and infrared spectra and become the most significant tracers of stellar atmospheres. Molecular bands, however, are much more complex than atomic lines and their fundamental constants are difficult to measure in the laboratory or to calculate ab-initio. On the other hand, molecular bands are very temperature sensitive and it is possible to measure fundamental molecular constants from astrophysical observations. Spectroscopic data for such measurements has become available through 10m class telescopes providing high quality data even for very faint stars. Molecular constants, once measured, will improve theoretical calculations of stellar atmospheres, and allow an accurate characterization of very cool M-star atmospheres.

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Interpretation of dynamo experiments

Contact: Andreas Tilgner

Several magnetohydrodynamics experiments are currently in operation or under construction (see the figure), which are intended to reproduce on a laboratory scale the mechanisms relevant to dynamo action in planetary cores or stars. All these experiments face the question as to what can be learnt from them about the geophysical or astrophysical problem they are supposed to explore. Numerical simulations are necessary in order to identify the characteristics common to experimental dynamos and dynamos occuring in nature. A possible topic of a PhD thesis is the investigation of the influence of turbulent fluid motion on dynamo action.

 

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Precession driven motion

Contact: Andreas Tilgner

The basic problem is delineated in the figure. The earth consists approximately of three layers: the inner core (IC), the outer core, and the mantle. Outer core and mantle meet at the core mantle boundary (CMB). The outer core, i.e. the region inbetween rIC and rCMB, is liquid. The motion of this liquid (mostly iron) is believed to be at the origin of the earth's magnetic field. The earth revolves once a day about the axis joining geographic north and south poles, and this axis itself precesses about the normal to the ecliptic which forms an angle of 23.5o with the diurnal rotation axis. The problem is to determine the fluid flow inside the outer core driven by the precessional motion of the solid boundaries. This flow could participate in the generation of the Earth's magnetic field. Another problem is the determination of the long term evolution of the earth-moon-sun system if the dissipation due to precession is taken into account.

The fluid motion driven by precession is also relevant for satellite dynamics, for instance when a manoever needs to be planned in which the spin axis of a spin stabilized satellite containing liquid fuel is reoriented.


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Turbulent convection

Contact: Andreas Tilgner

Turbulent convection is an ubiquitous phenomenon in geophysics and astrophysics, frequently in a rotating fluid or under the influence of a magnetic field. Even the simplest case of Rayleigh-Benard convection is not fully understood. The focus is at present on coherent large scale structures which persist in the presence of small scale turbulent fluctuations.


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